Since an exhaust system member such as an exhaust manifold for automobiles allows a high temperature exhaust gas which is emitted from an engine to pass, a material which constitutes the exhaust member needs a variety of characteristics such as high temperature strength, oxidation resistance, and thermal fatigue characteristics, and thus a ferritic stainless steel having an excellent heat resistance is employed for the material.
The exhaust gas temperature varies depending on the vehicle type, and, in recent years, is approximately 800 to 900° C. in many cases. The temperature of an exhaust manifold which allows a high temperature exhaust gas emitted from an engine to pass is as high as 750 to 850° C. With the emergence of environmental problems in recent years, further progression in strengthening exhaust gas regulations and improvement of fuel efficiency is proceeding. As the result, the exhaust gas temperature is believed to be elevated to about 1000° C.
Examples of a ferritic stainless steel which is used in recent years include SUS429 (JIS standard, Nb—Si-added steel) and SUS444 (JIS standard, Nb—Mo-added steel), which improve high temperature strength and oxidation resistance by addition of Nb as a principle element, Si, and Mo. However, SUS444 does not have sufficient high temperature strength and oxidation resistance for the temperature of an exhaust gas higher than 850° C. For this reason, a ferritic stainless steel having a temperature strength and oxidation resistance of SUS444 or higher is demanded. Herein, “oxidation resistance” is evaluated by increased amount of oxidation and the amount of spalled scale in a continuous oxidation test in the air, and it is assumed to be excellent when both the increased amount of oxidation and the amount of spalled scale are small. Since automobiles are used for a long period of time, oxidation resistance in cases in which a ferritic stainless steel is maintained at 1000° C. for 200 hours is needed.
For such a demand, a variety of materials for a exhaust system member have been developed. For example, Patent Documents 1 to 4 disclose a technique in which Cu—Mo—Nb—Mn—Si are added in combination. To the steel disclosed in Patent Document 1, Cu—Mo are added for the purpose of improving high temperature strength and toughness, and Mn is added for the purpose of improving scaling resistance. However, increased amount of oxidation is not clearly described, the conditions of a continuous oxidation test are 1000° C.×100 hours, and scale spalling ability in a case of exceeding 100 hours is not examined. Patent Document 2 discloses mutual adjustment of elements to be added for improving oxidation resistance of Cu-added steel. However, the temperature of the continuous oxidation test is not higher than 950° C., and a test at 1000° C. is not actually conducted. Patent Document 3 discloses a method in which repeated oxidation characteristics of steel is dramatically improved by optimizing the contents of Si and Mn. However, the total heat treatment time in the repeated oxidation test at the highest temperature is about 133 hours, and examination of oxidation resistance in a longer period of time has not been carried out. Although Patent Document 4 discloses a technique that high temperature strength and oxidation resistance are improved by adjusting the amounts of Mo and W, only the increased amount of oxidation is evaluated and the amount of spalled scale is not evaluated.
The present inventors disclose in Patent Document 5 a technique that Laves phase and ε-Cu phase are finely dispersed by adding Nb—Mo—Cu—Ti—B in combination to obtain high temperature strength at 850° C. The present inventors also disclose in Patent Document 6 a technique in which precipitation and coarsening of Laves phase are inhibited by making a carbonitride having Nb as a main phase fine in a Nb—Mo—Cu—Ti—B steel to obtain an excellent heat resistance at 950° C.